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EDIT AFTER COMMENTS

Ok, after given answers and comments I ended up with something like this:

template< typename Container >
std::string to_query_params(Container&& container)      // 1.
{
    if (container.empty()){
        return "";
    }
    std::string result;
    for( const auto& item : container ) {
        result.append("&")
              .append(std::get<0>(item))
              .append("=")
              .append(std::get<1>(item));               // 2.
    }
    assert(result.size() > 0);
    result[0]='?';
    return result;
}

• we're independent of underlying container, be it multimap, map, vector<tuple<string,string>>, etc;
• and we're independent of r/lvalueness of param being able to call to_query_params(generate_params());
• as of the container - I'll opt for std::vector<std::tuple<std::string, std::string>>, as we do not need to access elements, but iterate over them, so linear memory block will be cache-friendly;

2. • thank's to append we're not creating useless copies;

• std::stringstream is hudge and heavy, and I tend to avoid it as much as I can;

EDIT AFTER COMMENTS

Ok, after given answers and comments I ended up with something like this:

template< typename Container >
std::string to_query_params(Container&& container)      // 1.
{
    if (container.empty()){
        return "";
    }
    std::string result;
    for( const auto& item : container ) {
        result.append("&")
              .append(std::get<0>(item))
              .append("=")
              .append(std::get<1>(item));               // 2.
    }
    assert(result.size() > 0);
    result[0]='?';
    return result;
}

• we're independent of underlying container, be it multimap, map, vector<tuple<string,string>>, etc;
• and we're independent of r/lvalueness of param being able to call to_query_params(generate_params());
• as of the container - I'll opt for std::vector<std::tuple<std::string, std::string>>, as we do not need to access elements, but iterate over them, so linear memory block will be cache-friendly;

2. • thank's to append we're not creating useless copies;

• std::stringstream is hudge and heavy, and I tend to avoid it as much as I can;